1. Field of the Invention
The present invention relates to an optical system for a display panel, and a projector having the optical system. More particularly, the present invention relates to an optical system for a display panel for projecting red, green, and blue beams of a light source with high efficiency for light utilization.
2. Description of the Related Art
In conventional projectors using sequential color system, three lights of the primary colors, i.e., red (hereinafter, R), green (hereinafter, G), and blue (hereinafter, B) lights are sequentially projected onto a high response display panel by means of a color wheel that rotates at a high speed. A field image in each color is sequentially displayed, and a full color frame is displayed through the afterimage of human eyes. This kind of projector has the advantage that size, weight, and cost can be reduced easily because of the configuration of single display panel and a simple optical system. DMD (Digital Micro-mirror Device) is a known display panel. In a DMD, which is a binary-type device having only two states of ON and OFF, gradations of colors is controlled by controlling duties of ON/OFF through PWM (Pulse Width Modulation).
However, the optical system using a color wheel has the disadvantage that only one color can be used at one time, and two-thirds of light that is emitted from a light source is lost without being projected onto a display panel, resulting in the difficulty in improving brightness. If three primary colors of R, G, B into which white light of the light source is separated are used efficiently in a sequential color displaying process using a single display panel, it is possible to provide a projector with high brightness.
According to a disclosed optical system that addresses this requirement, light that is emitted from a light source is separated into color bands of R, G, B and a display panel is scanned with the color bands by continuously moving the color bands by means of a rotatable prism. See, for example, U.S. Pat. No. 5,410,370, and No. 5,528,318, Japanese Patent Laid-open Publication No. 2003-149738, and Japanese Patent No. 3352100. This system, called a scroll system, is considered to provide a projector with high efficiency for light utilization, because color beams of R, G, B which originate from the light source can be utilized simultaneously.
In conventional optical systems for a projector of a time-division type that uses a disk-shaped color wheel with color filters, out of the whole light consisting of color beams of R, G, B, approximately two-thirds can not be utilized simultaneously, as described above.
In the prior art scroll system disclosed in the above documents, in which color bands move continuously on a display panel, the color bands are narrow in width, and timing at which each color is switched differs for each pixel row. Therefore, a display panel driving circuit needs to be driven by a signal which is synchronized with color beams that has different switching timing for each pixel row. Further, each color band is projected onto a portion of the display panel only in a limited time, which makes it difficult to achieve sufficient gradations by the PWM. This means, in turn, that significantly complicated PWM control is required, and a heavy load is burdened on the signal processing circuit.